A size and weight of a solid polymer fuel cell that has a polymer electrolytic membrane can be easily reduced. Accordingly, practical applicability thereof as a power source for a mobile vehicle such as an electric vehicle or a small-size cogeneration system has been awaited.
The electrode reaction in the anode catalyst layer and in the cathode catalyst layer of a solid polymer fuel cell progresses at a triple phase boundary where reaction gases, catalysts, and fluorine-containing ion exchange resins (i.e., electrolytes) meet (hereafter referred to as a “reaction site”). Accordingly, a catalyst, such as a metal-supporting carbon catalyst that supports catalytic metals such as platinum on carbon black carriers with large specific surface areas and that is coated with a fluorine-containing ion exchange resin, which is the same with or different from the polymer electrolytic membrane, have been heretofore used as a constituent material for a catalyst layer of a solid polymer fuel cell.
Thus, proton and electron generation that takes place in the anode is carried out in the presence of the triple phase of catalysts, carbon particles, and electrolytes. Specifically, hydrogen gas is reduced in the presence of electrolytes that protons conduct, carbon particles that electrons conduct, and catalysts. Accordingly, the power generation efficiency increases as the amount of catalysts supported on carbon particles increases. The same applies to the cathode. Since catalysts used for fuel cells are noble metals such as platinum, the increased amount of catalysts supported on carbon particles disadvantageously increases the cost for producing fuel cells.
According to conventional methods for producing catalyst layers, an ink comprising an electrolyte such as Nafion® and catalyst powders of platinum, carbon, or the like dispersed in a solvent is casted and dried. Since catalyst powders are of several nms to several tens of nms, catalyst powders penetrate deep into the carbon carrier pores. In contrast, molecules of an electrolytic polymer are large and aggregated, and thus, an electrolytic polymer is deduced to be incapable of penetrating nano-sized pores and to merely cover the catalyst surface. Thus, platinum in the pores is not in full contact with the electrolytic polymer and it cannot be effectively utilized, which disadvantageously deteriorates catalyst performance.
JP Patent Publication (kokai) No. 2002-373662 A discloses a method for producing electrodes of a fuel cell wherein an electrode paste comprising catalyst-supporting particles comprising catalyst particles supported on the surfaces in combination with an ion-conducting polymer is treated with a solution comprising catalytic metal ions to subject the catalytic metal ions to ionic conversion into an ion-conducting polymer and then to reduce catalytic metal ions, for the purpose of improving power generation efficiency without increasing the amount of catalysts supported on carbon particles.
WO 2002/075831 discloses a solid polymer electrolyte-catalyst composite electrode which is composed of carbon particles supporting a solid polymer electrolyte and a catalytic material. The solid polymer fuel cell electrode contains carbon particles which are monolayer carbon nanohorn aggregates in which monolayer carbon nanohorns made up of monolayer carbon nanotubes of a unique structure having a conical shape at one end are aggregated into balls and a solid polymer fuel cell using the electrode, for the purpose of improving the efficiency of catalyst utilization for catalytic electrodes for a fuel cell.
WO 2002/075831 contains statements such that “carbon nanohorn . . . . When the aggregates are used as the carbon substances to constitute the solid polymer electrolyte-catalyst combined electrode, there may be provided secondary aggregates obtained by aggregating a plurality of the aggregates. Pores each having a size of several runs to tens nms exist between the secondary aggregates. Therefore, the combined electrode will have a porous structure. The pores effectively contribute to the channel of the reaction gas such as oxygen and hydrogen. When the secondary aggregates are formed, the catalytic material can be carried to inside the secondary aggregates, and the solid polymer electrode can penetrate into the secondary aggregates, thereby providing excellent catalytic efficiency.” This document also contains statements such that “at least a part of the carbon molecule aggregates or the carbon nano-horn aggregates 10 has an incomplete part. The term “incomplete part” herein means a broken structural part. For example, a carbon-carbon bond in a six-member ring is partly cut, or a carbon atom therein is lost, which constitutes the carbon molecule or the carbon nano-horn 5. A vacancy or a bond with other kind of a molecule may be formed. The above-mentioned incomplete part may be large and expanded to such an extent that it is referred to as a hole in the carbon six-member ring. Each of them herein refers the “pore”. The pore may have, but not especially limited thereto, a diameter of about 0.3 to 5 nm, although the pore diameter is not particularly limited.”
Also, JP Patent Publication (kokai) No. 2004-152489 A discloses an invention wherein a carbon nanohorn aggregate is used as a carbon material for use in a catalyst layer of a catalyst supporting carbon particle, a solution of a metallic salt, and the carbon nanohorn aggregate are mixed, a reducing agent is added and mixed with agitation, a catalytic metal is supported on the surface of the carbon nanohorn aggregate, and a reducing-treatment is performed at a low temperature to regulate a particle diameter of the catalyst metal, for the purpose of improving the efficiency of catalyst utilization for catalytic electrodes for a fuel cell.